This invention relates to the monitoring of a telecommunications system for faults. Telecommunications networks have a large number of components distributed over a wide area, and it is important to be able to identify faults when they occur and to deal with them promptly before they manifest themselves to the users of the network as a service interruption. The user of the network, in turn, may have a service level agreement with the network operator which specifies contractual penalties if service interruptions exceed a predetermined limit. Many methods are known for identifying faults and alerting the operators to them so that remedial action can be taken. Included in the term `fault` for the purposes of this specification are not only equipment or service breakdowns but events such as overloads which may be due to external causes but require remedial action.
Different users of a system need notification in different circumstances. A telecommunications network operator, responsible for monitoring equipment, needs to know of failures of individual equipment. However, it is possible for an item of equipment in a telecommunications network to fail with no immediate effect on the level of service provided to a customer, if for example alternative routings are available, or if the customer is not using the full capacity of the system. Conversely a customer may experience a service interruption when no individual item of equipment has failed, for example if the total demand for use of the system by all customers exceeds the capacity of the system. In order to monitor the service supplied to its customers, the network operator also needs to be informed of such service interruptions.
For example, in a telecommunications network, if a channel is running at full capacity any further call attempts on this channel would fail. If the rate of occurrence of such call failures increases, this indicates that the network is at full capacity for an increasing proportion of the time. Remedial action may be possible, for example by reallocating capacity from elsewhere (e.g. a second channel in a time division multiple access system). In the case of service interruptions to a customer, these can be logged. The nature of the service level agreement will determine what information is required, but in most cases a guaranteed minimum level is agreed. The minimum level may be an agreed proportion of time that the service is available. In other circumstances, for example where re-start procedures are complex, the number of separate interruptions may be used as an additional or alternative criterion.
There is a difficulty in fault monitoring, that phenomena which may be of a transient nature may be indicative of the presence of an actual or imminent underlying problem, but may instead be no more than statistical fluctuations of no significance. A transient fault which appears to clear itself may signify that the cause has ceased, or it may signify that there is a underlying problem and the fault will recur under certain conditions. A particular special case of this problem occurs when an alarm is triggered by a property of the system exceeding a predetermined threshold value. If the threshold value is set too high the alarm indicator will not be triggered until the situation is already critical, leaving no margin in which to take remedial action. However, if the threshold is set too low there will be an excessive number of false alarm indications. The importance of transient faults may differ according to individual customers' requirements. To a customer whose terminal equipment includes error-correction facilities intermittent, transient, faults may be unimportant. However, to a customer for whom any interruption, however short, necessitates a re-start operation involving complex security procedures or a site visit to re-set equipment, a large number of transient interruptions are much worse than a single interruption of longer length.
Transient faults have to be handled differently from non-transient faults. Transient faults clear themselves, so there is no need to take any remedial action to clear them manually. However, because they clear themselves, it is difficult to investigate the cause of a transient fault, or to identify patterns which may indicate an underlying problem.
A prior art fault-monitoring system is known from IBM Technical Disclosure Bulletin No. 7 (December 1992). Each second, this system records whether or not a fault is present, by detecting transmission errors. An alert is sent if the number X of individual seconds in which a fault was detected in a period of 15 minutes exceeds a first threshold L or if in a period of 24 hours the number Z of individual seconds in which a fault was detected exceeds a second threshold which is, in proportion to the size of the measurement period, much lower than L. This arrangement allows statistically significant changes in fault rate to be detected for both long and short periods allowing sudden large changes to be detected promptly, and also detecting smaller long-term changes which, because of the larger sample size, are statistically significant, without false alerts caused by normal short-term statistical fluctuations.
This prior art system only monitors the number or seconds in which a fault is present. It takes no account of the duration of the individual faults. For example, a report of nine "errored seconds" in a 15 minute period could be caused by nine individual faults each of less than ore second, or a single fault of nine seconds. It also makes no provision for separate faults occurring simultaneously, or of overlapping duration; only a maximum of one fault per second can be recorded.